Abstract

Stable sensors for low-temperature applications are realized from donor-doped $BaTiO_3$ polycrystalline ceramics having a strong negative temperature coefficient of resistivity (NTCR) below the orthorhombic-rhombohedral phase-transition point, from $10^2-10^3 \hspace{2mm}\Omega$ cm at 190 K to $10^{10}-10^{13}\hspace{2mm} \Omega$ cm at $\approx$ 50 K, and a thermal coefficient of resistance, $\alpha =20-23\%\hspace{2mm} K^{-1}$. The temperature region of the NTCR can be modified by substituting isovalent ions in the lattice. Highly non- linear current-voltage (I-V) curves are observed at low temperatures, with a voltage maximum followed by the negative differential resistance. The I-V curves are sensitive to dissipation, so that cryogenic sensors can be fabricated for liquid-level control, flow-rate monitoring, radiation detection or in-rush voltage limiting. The observed NTCR of $n-BaTi0_3$ ceramics at low temperatures is made use of in evaluating the composition of aliphatic hydrocarbon (petrol)-alcohol mixtures through the determination of freezing points, since these liquid mixtures are used as an alternative fuel in internal combustion engines.